Power converters are used in aircraft electrical power systems as well as in power systems for other apparatus. The electrical power systems on current commercial aircraft are primarily provided by 400 Hz, three-phase 115V or 230V AC power sources. The power system may include one or more alternative low voltage DC power sources, such as a fuel cell stack or a battery, which provides input power to a pulse width modulated (PWM) power conversion system. Multiphase power, such as three-phase power, may be provided to an aircraft electric power distribution system, which, in turn, provides the electrical power to a downstream distribution system. The downstream distribution system may have loads of various types, including, but not limited to, three-phase, single-phase, or another conversion system with DC loads, etc.
A problem with such multiphase power systems is that they may be presented with different types of loads. In three-phase systems, such loads include 1) three-phase balanced linear loads, 2) three-phase unbalanced linear loads, 3) three-phase balanced nonlinear loads, and 4) three phase unbalanced nonlinear loads. Disturbances in the supply output voltages occur when driving nonlinear or unbalanced loads. The disturbances often include specific higher harmonics of the fundamental frequency of the power system. Achieving fast dynamic response while concurrently meeting total harmonic distortion (THD) requirements becomes problematic.
Repetitive control has been used in an attempt to reduce the presence of such harmonics. This type of control attempts to provide asymptotic output tracking of periodic portions of the supply output voltages while rejecting the periodic harmonic disturbances that are present. Generally stated, it is based on the Internal Model Principal and used to deal with periodic signals, for example, tracking periodic reference signals or rejecting periodic disturbances. The repetitive controller may incorporate an internal model, and a local positive feedback of a delay line cascaded with a low-pass filter, to track or reject periodic signals. Additional compensation may be used for the system stability purpose.
Although repetitive controllers are used to provide rejection of harmonics in power systems, they often have a slow dynamic response. High performance in both transient and steady-state may therefore be difficult to achieve using repetitive controllers.